By Allen Austin, ABB Inc.
The evolution of high-efficiency solar photovoltaic (PV) string inverters is driving a shift toward higher AC voltages in utility-scale solar applications. Using string inverters in solar plants rated at 20 megawatts and below can be extremely beneficial. With output voltages now as high as 1000VAC, the same amperages will yield higher output power levels.
Solar PV string inverters were initially used primarily in residential and commercial building applications up to 1MW in size. This was limited to traditional AC voltages of 120VAC single phase and 480VAC three phase. String inverters are well suited to rooftop solar applications due to their relatively small size and ease of installation for inverters rated 100KW and below. So, string inverters could be easily connected to standard building cabling and existing rated wiring.
Benefits of going to higher voltage
A typical string inverter rated 100KW at 480VAC will yield approximately 208amps. New string inverters rated 100KW at 1000VAC will yield approximately 100amps. This reduction in amperage reduces heat in cabling and reduces the required diameter of the wire resulting in cost savings. Cable connectors can also be smaller for easier wiring and connection.
Because string inverters are much smaller and lighter than large 1MW+ central inverter skids, no heavy equipment is needed for installation. This is especially important in locations that cannot be accessed by the tractor trailers and cranes needed to install central inverter skids. By contrast, string inverters can be transported to site using standard utility trucks. Installations are normally performed by two technicians and can be mounted directly to the PV array structure.
Using string inverters also allows for increased monitoring at the PV array level.
Figure 1 shows a line diagram of a typical string inverter application at utility scale, specifically highlighting the AC panels/combiners.
Figure 1: a typical utility-scale string inverter application
Challenges in using higher AC rated string inverters in utility scale solar PV applications
As illustrated in the above diagram, the outputs from string inverters are summed together in an AC panel board also known as an AC combiner or re-combiner. In this case, all the connection components will have to be rated at or above the output voltage and current of the string inverters. In addition, the protection components such as breakers must have a high enough kiloamp interrupting capacity (KAIC) rating in order to protect the system from a short circuit condition downstream.
The AC combiner is normally connected to a step-up transformer rated 34.5 KVA on the secondary side, which implies a breaker rating of 35-45 KAIC. Higher string inverter output voltages requires higher-KAIC rated components in the AC combiner box.
In Europe, IEC-rated products are prevalent and there are several components that have the IEC required KAIC ratings at 1000VAC and 800VAC. By contrast, in the US today there are only a few UL-rated components on the market at the 1000 VAC level. Several are available at 800VAC and more at 600VAC.
Below are some of the components available to AC panel board and AC combiner OEMs working with UL systems.
Disconnect switches used for isolation and maintenance
600-1000v fused and non-fused up to 800a
Molded Case Circuit Breakers used for protection, isolation and monitoring. Input from string inverters
600-800v, 250a to 400a up to 35 KAIC
Air Circuit Breakers used for main circuit protection and monitoring after inverter inputs are paralleled
600v, 3,200a at 40 KAIC
800/900v, 3,200a at 40 KAIC (IEC only)
Contactors used for remote switching
600v, 25a to 1350a
1000v, 60a to 1650a
AC panel and combiner box builders can rest assured that they are integrating the correct and state of the art components to meet the needs of their customers choosing to use solar PV string inverters for their utility scale solar PV plants.
In addition to the higher voltage levels, PV systems are also being fully integrated with cloud-based measuring and monitoring tools. Including components that support this level of functionality (e.g., the ABB Ability™ platform) as part of system design will ensure better efficiency and durability.